Hyperbaric Oxygen Therapy for Horses

Human doctors have used hyperbaric oxygen therapy to help treat everything from wounds to inflammation, but its efficacy in horses is not as well-documented

Many new therapies and medical treatments that are developed for use in humans are rapidly adapted for use in horses. A number of veterinarians across the United States have adopted one of these, hyperbaric oxygen therapy (or HBOT), for treating everything from chronic wounds in horses to infertility. Scientists originally developed this modality to treat scuba divers with decompression sickness ("the bends") and have studied HBOT use for many human conditions for more than 20 years. Its use in animals, however, is often controversial. In this article we'll discuss the theories behind HBOT and evidence both for and against its use in equine medicine.

What it is and How it Works
During hyperbaric oxygen therapy treatments the equine patient is placed in a large metal chamber that is then pressurized up to two or sometimes three times the normal atmospheric pressure. The air is replaced with high concentrations of pure oxygen, and the patient remains in this environment for an hour or more before the chamber operator depressurizes the unit. Depending on the reason for treatment, physicians or veterinarians usually prescribe multiple treatment sessions, with human patients undergoing anywhere from 10 to 14 treatments for minor conditions to more than 50 HBOT treatments for severe conditions such as bone infections.1 Horses suffering from serious conditions might undergo anywhere from two to 30 treatments, depending on the condition. Horses being treated as part of a health maintenance program generally need fewer sessions overall. For instance, some veterinarians believe racehorses will have a shorter recovery time after a racing effort if HBOT is integrated into their training and racing programs.

Steve Adair, MS, DVM, Dipl. ACVS, ACVSMR, associate professor of equine surgery at the University of Tennessee's College of Veterinary Medicine, says a normal equine treatment session begins with a 15- to 20-minute period during which the pressure is raised from 1 atmosphere to 2.5 atmospheres. (Atmospheric pressure at sea level is equal to 1 atmosphere, and exposure to higher pressure would be similar to what a person would experience under water; each 33 feet--10 meters--of sea water provides an equivalent increase of 1 atmosphere.) Next, the chamber operator maintains the unit at the elevated pressure for 60 minutes. At the end of one hour, the chamber operator slowly (over another 15 or 20 minutes) brings the unit back to normal atmospheric pressure.

So how does HBOT work? Veterinarians and physicians who prescribe the therapy explain that the elevated pressure and oxygen concentration in the chamber combine to force more oxygen into the patient's bloodstream with each breath. This increases the amount of oxygen dissolved in the blood and is thought to make oxygen more available to injured tissues. They say this can promote healing, especially in large wounds where blood supply has been compromised. One study has shown that it causes the inflammatory cells to form free radicals (which these inflammatory cells need in order to function properly and, thus, kill harmful microorganisms) and increases the efficacy of certain antibiotics; for these reasons, HBOT is often used to treat long-standing infections.

How it is Used: Humans vs. Horses

The Undersea and Hyperbaric Medical Society (UHMS) guides clinical HBOT use in humans and has approved the therapy for treating 14 specific conditions, ranging from carbon monoxide poisoning to chronic infections to acute treatment of certain types of injuries. In addition to supplying increased oxygen levels to damaged tissues, helping the immune system, and assisting certain antibiotics, veterinarians and physicians who employ HBOT note it has several other methods of action: With carbon monoxide poisoning, for instance, prescribers use HBOT to "force" carbon monoxide out of the body; oxygen competes for binding sites on red blood cell hemoglobin molecules that are responsible for oxygen transport. Prescribers also use the approach to reduce swelling in acute injury. When organ tissues are metabolically active (e.g., exercising muscle) they produce substances that tell tiny blood vessels within the tissues to dilate, increasing blood supply and thus oxygen delivery to the injured area. On the other hand, when the oxygen supply exceeds the tissues' needs, the vessels constrict, restricting blood flow, which helps prevent swelling and future tissue injury. Thus, treating recently injured tissues with HBOT promotes this vasoconstriction while maintaining an adequate oxygen supply to the tissues.

Researchers have conducted extensive studies on HBOT's effectiveness for treating the 14 UHMS-specified conditions in humans, and our limits of tolerance are well-defined. The Veterinary Hyperbaric Medicine Society, based at the University of Tennessee, promotes HBOT use in veterinary practice, but in contrast to the large amount of data regarding HBOT use in humans, only three published studies on its effects in horses are available. In the first study, published in 2008 in the American Journal of Veterinary Research, researchers found that HBOT increased inflammation and impaired new blood vessel formation in otherwise healthy skin grafts (see TheHorse.com/11185). In the second study, published in the same journal in 2011, veterinarians examined the effects of HBOT on signs of endotoxemia and found that while it improved the investigators' subjective assessment of the horses' health, it did not significantly affect any of their objective measurements (heart rate or temperature). Both studies' authors concluded that HBOT was not recommended as a treatment for the particular cases investigated. Despite this, some equine HBOT practitioners recommend its use for virtually all common equine ailments. "In most cases it's adjunctive or supportive," says Adair. Nevertheless, he says he has seen good results in treating slow-healing and infected wounds with HBOT.

On Feb. 10 a hyperbaric oxygen therapy (HBOT) chamber at the Kentucky Equine Sports Medicine and Rehabilitation Center (KESMARC) in Ocala, Fla., exploded after a horse's steel shoe made contact with a metal surface inside the chamber. The horse, 6-year-old Thoroughbred gelding event horse Landmark's Legendary Affaire, as well as staff member Erica Marshall were killed; another woman, Sorcha Moneley, was injured. Thirty other horses contained in the building where the chamber was located were not harmed in the incident.

In light of the tragic accident, Dennis Geiser, DVM, Dipl. AVBP, associate professor in the Large Animal Clinical Sciences department at the University of Tennessee's College of Veterinary Medicine and founder of the Veterinary Hyperbaric Medical Society, says practitioners strive to minimize risk associated with the technology.

While he declined to comment specifically about the KESMARC Florida incident, Geiser says that HBOT chambers used in the United States are manufactured according to specific safety standards, and that practitioners take precautions to ensure horses' safety during treatment.

Geiser explains that while the oxygen itself is unlikely to ignite, oxygen under pressure could ignite a fuel source (e.g., hair) in the presence of a spark. "So we don't put shod horses in the chamber," Geiser says of his approach to safety. "We take the shoes off if we can, or we put boots on the horse or otherwise cover its feet."

In addition, he says rubber material runs about halfway up many chambers' interior walls to further minimize spark risk. According to reports, the KESMARC chamber did have protective padding, but a section was damaged by the horse's kicking, revealing metal.

--Pat Raia

"We've also seen improvement in chronic laminitis," he adds. "Subjectively, it seems to improve hoof wall growth after 20 or 30 treatments."

In a third study published this year, in the Equine Veterinary Journal, researchers at the University of Tennessee examined how to optimize the use of peripheral blood as a stem cell source. They reported that it significantly increased stem cell population in the peripheral circulation after three treatments.

The Data
To assess any new treatment, one must first objectively evaluate the treatment's effectiveness by examining several parameters of healing. With HBOT, these include evidence that the therapy 1) elevates the amount of oxygen dissolved in the blood (achieving a state called hyperoxia); 2) increases the amount of oxygen present in the injured tissue; and 3) improves the ultimate outcome of the disease or injury that's being treated.

A large number of studies performed in humans and animals have revealed a significant elevation in blood oxygen levels with HBOT, a phenomenon that is well-understood and generally accepted. Fewer studies are available on the second parameter, oxygen delivery to the tissue, due to the inherent difficulties of measuring tissue oxygenation. Nevertheless, several experimental studies as well as computer models have suggested HBOT increases tissue oxygenation, at least in certain tissues. However, despite some equine HBOT providers' assertions that the modality increases blood flow to the injured tissue, "The only mechanism by which oxygen delivery is increased is by increasing the dissolved oxygen in the blood," says Eugene Bruce, PhD, professor and pulmonary gas exchange and respiratory physiology researcher at the University of Kentucky.

He says the volume of blood flowing to a particular region of the body generally does not increase in cases of hyperoxia, although the various compensatory mechanisms that hyperoxia and hyperbaric pressure activate make it difficult to predict what will happen in any given tissue. There is also evidence that HBOT does not affect all organs equally, as researchers detected a 50% decrease in blood flow to the heart in treated rats but no blood flow change to the liver or kidneys.

"There is a subtle interplay between the effects of carbon dioxide on blood flows (the horse inhales less during HBOT, causing arterial carbon dioxide to rise) and those of oxygen, and it is unclear whether the same balance of these effects is achieved in all tissue beds," says Bruce. Therefore, researchers might see different treatment results depending on the tissue, disease, and treated animal species.

Evaluating treatment outcome is the third way to assess HBOT. Doing this objectively can be difficult due to the limited published equine HBOT research available for comparison; Adair says this scarcity is due to the inherent difficulties of measuring physiological variables and collecting samples from horses in the chamber. He and other researchers at the University of Tennessee are currently working to develop methods that would allow them to collect blood from horses in an HBOT chamber.

A Word of Caution
Like most treatments, HBOT is not without side effects. Haim Bitterman, MD, of Technion Israel Institute of Technology, noted that the most common patient complaint is middle ear trauma resulting from the elevated pressure. In fact, one study found that up to 45% of human patients showed signs of eustachian tube dysfunction during treatment. Other problems, such as changes in vision, lung damage and edema (fluid swelling), and seizures, were less common and usually went away when treatment stopped.1

The most serious potential side effect of HBOT in humans is oxygen poisoning. The lungs, which are exposed to higher oxygen levels than any other internal organs, are affected first. Consequently, changes in lung function can make it more difficult for the patient to breathe. These changes often go unnoticed (are latent) during or immediately after treatment, but they might become apparent several hours later and can be accompanied by coughing, pain during inspiration, and a burning sensation in the chest, says Bitterman. While long (more than five hours) exposures to hyperbaric oxygen treatment are usually required to induce oxygen poisoning, University of Pennsylvania researchers noted a wide range of tolerance levels in humans. These authors emphasized the need to follow strict guidelines for use and continuously monitor the patient during therapy to allow early detection of oxygen poisoning.

The other major organ system oxygen poisoning can affect is the central nervous system, which, like the lungs, can sustain damage that's latent at first. Signs in people ranging from dizziness and nausea to seizures might arise without warning but are usually seen if exposed to pressures of 2 atmospheres for 4+ hours or 3 atmospheres for 3+ hours (Lambertsen CJ, Clark JM, Gelfand R, et al, 1987). For this reason, Adair does not recommend HBOT for horses with any condition (e.g., a fever) that would lower the animal's threshold for developing seizures. This also is one of the reasons operators, as a standard procedure, check each horse's temperature immediately prior to placing him in the chamber. While mandated safety guidelines have not been established for HBOT in horses, most technicians treat horses in accordance with recommendations for humans. In particular, Adair noted that after treating several hundred horses, he believes HBOT in the range of 2 to 3 atmospheres of pressure can be used as safely in horses as it is in humans.

Take-Home Message
While hyperbaric oxygen therapy might benefit patients with certain injuries or illnesses, veterinarians must conduct additional clinical trials to objectively assess this therapy's benefits and to establish equine-specific safety guidelines and exposure limits. Researchers' understanding of this therapy's impact on horses--like all emerging therapies and technologies--is still a work in progress, and although veterinarians using HBOT say the modality shows significant promise, they emphasize it is not a cure for all diseases.

Featured Adoptable Horse

Stella

She's a beautiful filly but I can't keep her and we can't keep up with the costs,she loves to be petted and is still nervous around water since the "accident", I've had to bottle feed her and she's halter trained and we are letting her get used t ... Read More